The present invention relates to a controller and control method of a reducing agent injector used for an exhaust gas purification apparatus. Particularly, the present invention relates to a controller and control method of a reducing agent injector for injecting reducing agent into an exhaust gas passageway of an internal combustion engine in order to reduce nitrogen oxide contained in exhaust gas.
Conventionally, exhaust gas exhausted from an internal combustion engine, such as a diesel engine, contains nitrogen oxide (NOX) that may cause environmental pollution. As an exhaust gas purification apparatus used for reducing and purifying this NOX, a selective catalytic reduction (SCR) system that uses urea solution as a reducing agent is known. The SCR system injects aqueous solution of urea into an exhaust gas passageway on the upstream side from reduction catalyst to cause ammonia produced from the aqueous solution of urea to act with NOX contained in exhaust gas in the reduction catalyst, decomposing NOX into nitrogen, water, carbon dioxide and the like to be released into atmosphere.
In such an SCR system, when the amount of the reducing agent to be injected into an exhaust pipe is excess or deficient with respect to the amount of NOX, some NOX remaining unreduced may be released into atmosphere or produced ammonia may slip into the downstream side from the reduction catalyst. In order to avoid such reduction in purification efficiency of NOX and ammonia slip, the SCR system is required to inject into the exhaust pipe just the right amount of reducing agent predetermined according to the amount of NOX.
As a device for feeding reducing agent in the SCR system, for example, an electromagnetic driven reducing agent injector is known in which a controller controls movement of a valve needle by controlling energization of an electromagnetic solenoid to control opening and closing of an injection hole. This reducing agent injector is subject to the energization control based on a target injection duration determined depending on a target injection amount of reducing agent determined by calculation. Specifically, injection start time and injection duration are controlled by controlling the ON-OFF DUTY ratio (ratio of ON-duration to one cycle of pulse wave of PWM signal) of a switching device provided in a circuit for controlling the reducing agent injector (see Patent Document 1).
In such an electromagnetic driven injector, while holding the valve needle open requires energization with only a relatively small amount of current, quickly moving the valve needle from the closed state to open the injection hole requires a relatively large amount of current.
Controlling opening and closing of an injector by PWM controlling a switching device is applied to a fuel injector for injecting fuel into a cylinder of an internal combustion engine as well as a reducing agent injector. Among those applications, methods for controlling a fuel injector provided to a diesel engine includes a control method for moving a valve needle quickly to ensure good controllability of a fuel injector by causing a first current to flow that is relatively large in the beginning of an injection period and then causing a second current to flow that is relatively small but enough to hold the valve open (see JP-A-2008-180101 (paragraph [0022]) and JP-A-2008-190345 (entire text and drawings)).